Process for obtaining magnesium oxide from dolomitic rock



Patented May 24, 1938 "UNITED STATES PATENT. OFFICE No Drawing. Application February 15, 1937, Serial N0. 125,930

, v 8 Claims.

This invention relates to the production of substantially pure magnesium oxideand has for its object to obtain a product of this character from dolomitic rock, such as dolomite or dolomitic limestone, at a minimum cost.

In practicing the invention dolomite or dolomitic limestoneis calcined to produce a mixture of substantially three parts CaCOa and one part MgO and a" minimal amount of CaO. This soft calcine is comminuted and'a cold water suspension of the three components of the-mixture is then formed. Thesuspension is first made by the addition to the calcine of about one-fourth of the final volume of. added water'to effect hydration of the MgO of the calcine. The hydration maybe accomplished by aging the suspension but preferably is'eXpedited by application of heat. The remaining three-fourths of the full quantity of water is then added, the preferred final ratio of calcine to water being within the range of 1 to 16 and 1 to 20. Such a suspension haspractically'no solvent action on the carbonate and MgO solids which are merely held in suspension in the cold water. Into this suspension hydrogen sulfide gas (HzS) is injected. The hydrogen sulfide gas combines with the hydrated MgO in suspension to produce a cold solution of MgHzSz, the CaCOz remaining as a solid suspended in this solution of MgHzSz. This cold solution of MgHzSz is then separated from'the' inert calcium carbonate (CaCOa) by filtration, centrifuging or otherwise and a clarified cold water solution of MgHZSZ is obtained. This clarified solution is then heated to the boiling point, preferably by first injecting steam into the solution and then applying extraneous heat in any suitable way. This heat causes the MgHzSa solution to break down into solid hydrated magnesia (Mg(OH)2) and the gas. Has. The heating of the solution is carried'on in a closed chamher, to the complete evolution of H23 so that this HzS gas is recovered for use in repeating the operation.

As thus far described,the small amount of caustic, or free lime in the calcine mixture has been ignored, and the final MgO product, while being of high quality, is not absolutely pure, since the free lime would also react with and be dis-- solved by the injected gas, H28, and the ,resultant hydrated MgO product would be contaminated thereby. To produce pure MgO the small free lime content of the calcine mixture (MgO,

, CaCOa and some CaO), is eliminated from the cold water suspension before injection of the H25 gas. This elimination of active or free verted to either calcium carbonate or' sulfate in the suspensioml-lzS gas is injected into this limefree cold suspension until the H28 gas comes to a pseudoequilibriuin with the cold suspension, at which point about %-90% of the MgO content of the suspension will be found to have entered into solution. When this point is reached, if there is further injection of the HzS gas, said gas will pass on-through the suspension, without being absorbed. When the point is reached where no further absorption of H28 gas takes place,.the solution is permitted to stand for a few hours, say over night, after which additional HzS gas is injected until no more will be absorbed. This results in a. practically. complete solution of the MgO content of the calcine in the suspension. When such practically complete solution of the initial MgO content of the calcine has been secured, the solutionv is clarified as above described and then subjected to heating, and it breaks down into substantially pure solid hydrated magnesia (Mg(OH)2) and H28 gas, the gas being recovered and held ready for use in repeating the operation.

The resultant precipitate, after filtration or. centrifuging, may then be dried and lightly calcined in asingleoperation to convert the hydrated oxide, Mg(OH)2 to the oxide, MgO.

Having thus described the invention, what is. claimed is:

1. A process which consists in forming a cold water suspension of a comminuted dolomitic calcine comprising CaCOs and MgO, permitting hydration of the suspended MgO to ensue, then bringing the hydrated MgO content of the calcine into solution by injecting hydrogen sulfide gas into the suspension,- clarifying the solution and boiling the clarified solution. I

' j 2. A process which consists in forming a col drogen-sulfide-impregnated water, clarifying the. 56

solution and heating the clarified solution to precipitate hydrated magnesium oxide, and converting the Mg(OHz) to MgO by heat.

3. A process which consists in forming a cold water suspension of a comminuted dolomitic calcine comprising CaCOa and MgO, permitting hydration of the suspended MgO to ensue, eliminating the CaO content of the calcine from the suspension, subjecting the hydrated magnesium oxide in the suspension to treatment with H2S to cause the Mg(OH)2 to pass into solution in the cold hydrogen-sulfide-impregnated water, clarifying the solution and heating it to precipitate hydrated magnesium oxide, and converting the Mg(OH)2 to MgO by heat.

4. A process which consists in forming a cold water suspension of a comminuted dolomitic calcine comprising CaCOa and MgO, permitting hydration of the suspended MgO to ensue, converting any "free lime ((3210) in the suspension to calcium carbonate, (CaCOs), then injecting hydrogen sulfide gas (HzS) into the suspension, whereby the hydrated magnesium oxide (Mg(OH)2) in the suspension is brought into solution in the cold hydrogen-sulfide-impregnated water, clarifying the solution by removing solids therefrom, then heating the clarified solution and collecting the resultant hydrated magnesium oxide and hydrogen sulfide gas, and converting the Mg(OH) 2 to MgO by heat.

5. A process which consists in forming a cold water suspension of a comminuted dolomitic calcine comprising CaCOa and MgO, permitting hydration of the suspended MgO to ensue, converting any free lime. (C80) in the suspension to calcium sulfate (CaSOii), then injecting hydrogen sulfide gas (H28) into the suspension, whereby the hydrated magnesium oxide (Mg(OH) 2) in the suspension is brought into solution in the cold hydrogen-sulfide-impregnated water, clarifying the solution by removing solids therefrom, then heating the clarified solution and collecting the resultant hydrated magnesium oxide and hydrogen sulfide gas and converting the Mg(OH) 2 to MgO by heat.

6. A process which consists in forming a cold water suspension oi. a comminuted dolomitic calcine comprising CaCOa and MgO, permitting hydration of the suspended MgO to ensue, eliminating any free lime from the suspension, then injecting hydrogen sulfide gas (HzS) into the suspension until no more of the injected gas is absorbed thereby, then permitting the suspension to stand for a short period of time, then again injecting hydrogen sulfide gas into the suspension, whereby the hydrated magnesium oxide (Mg(OH)z) in the suspension is dissolved in the cold hydrogen-sulfide-impregnated water, clarifying the solution by removing solids therefrom, and then boiling the clarified solution and collecting the resultant hydrated magnesium oxide and hydrogen sulfide gas and lightly calcining the Mg(OH)z to produce MgO.

7. A process which consists in forming a cold water suspension of a comminuted dolomitic calcine comprising CaCOa and MgO, permitting hydration of the suspended MgO to ensue, converting any free lime of the suspension into calcium carbonate, injecting hydrogen sulfide gas into the suspension, whereby a cold solution of MgHzSz is formed with calcium carbonate as a solid suspended therein, eliminating the calcium carbonate from the MgHzSz solution, then boiling the solution, collecting the resultant hydrated magnesium oxide and the hydrogen sulfide gas and lightly calcining the recovered Mg(OH)z to produce MgO.

8. The process of obtaining an approximately pure magnesium oxide which consists in forming in cold water a suspension of dolomitic calcine comprising CaCOa and MgO by first adding to the calcine only approximately enough cold water to hydrate the MgO and then adding enough cold water to complete the suspension, then subjecting the hydrated magnesium oxide of the calcine suspension to treatment with Has to cause the MgiOH): to pass into solution in the cold hydrogen-sulfide-impregnated water, clarifying the solution and heating it to precipitate hydrated magnesium oxide, and converting the Mg(0H)2 to MgO by heat.

WALTER H. MACIN'I'IRE. 

